1. Field of the Invention
This invention relates to cable constructions designed for the transmission of power or communications, with more specific reference to improved cable constructions wherein a multilayer metal element of substantially pure aluminum bonded to an aluminum alloy is utilized as a shield therein to provide protective electrical and mechanical properties, particularly good galvanic corrosion protection.
2. Description of the Prior Art
In general, the design of cables, whether for the transmission of power or communications, is dependent on their use in aerial, submarine, or underground service and in high or low voltage or frequency applications. The use of metallic shields in such cable constructions is well known. In cable designs, the outer shield or return conductor has many functions to perform. First, it has to carry, or be capable of carrying in the case of a malfunction or emergency, an electrical current. The source of this current might be the charging current due to capacitance of the cable, an out of balance current in a three-phase system, a current due to a fault in the cable, or a current initiated by a lightning stroke. The electrical characteristics of the shield are dictated by the overall service requirements of the cable. Usually, these electrical service requirements will dominate other requirements of the cable.
In addition, the outer shield must provide some mechanical protection to that part of the cable which provides the electrical service for which it is designed. For example, a cable may be subjected to mechanical damage while it is being installed, particularly in the case of an underground cable being laid by machine. Also, rocks in the backfill may exert excessive local pressures on an underground cable or the cable may be damaged in the course of subsequent digging near the same. Another important hazard to which underground cables are exposed and must be protected against by the outer shield is the gnawing of rodents. Finally, the outer shield should provide an hermetic seal to guard against the entrance of moisture into the working parts of the cable.
In many cases, where the primary function of the cable is not critical or where the cable can be installed in a protective environment, a single material can be found to provide a satisfactory shield. In other cases, the electrical and mechanical requirements noted above are mutually conflicting. For example, relatively pure copper or aluminum are excellent materials for satisfying the electrical requirements of the outer shield because of their high electrical conductivity, but do not satisfy the mechanical requirements since they are soft and ductile in their pure form. In like manner, the material most commonly used for mechanical protection in the shield is steel since it can be made very strong, but it will not satisfy the electrical requirements of the outer shield because of its low electrical conductivity. Under these circumstances, composite shields of aluminum or copper and steel have been used as illustrated in U.S. Pat. Nos. 3,183,300 and 3,272,911. Unfortunately, steel has little inherent resistance against corrosion unless very costly nickel or manganese alloys are used. Furthermore, composite shields of aluminum or copper and steel lead to additional problems of bi-metallic galvanic corrosion which is the result of interfacing metals having widely differing electrolytic potentials as in the case of aluminum or copper and steel.